Line a baking sheet with a clean kitchen towel or a few layers of paper towels; set aside. Working in batches, cook cabbage leaves in a large pot of boiling generously salted water until bright green and pliable, about 2 minutes per batch. Transfer leaves to a bowl of ice water to cool; reserve pot of water for cooking rice. Transfer cabbage leaves to prepared baking sheet and let drain.

Return water in pot to a boil and cook rice, stirring often, until grains swell and rise to the surface, 3–6 minutes. Drain rice and rinse under cold running water to stop it from cooking further. Drain again and transfer to a large bowl.

Wipe out pot. Pour in ¼ cup oil and set pot over medium heat. Add onion and cook, stirring occasionally, until softened and golden, 7–9 minutes. Add pine nuts and cook, stirring often, until nuts smell toasty and have slightly darkened in color and onion is almost jammy, about 5 minutes. Mix in herbs, raisins, and 2 Tbsp. sumac and cook, stirring, until fragrant, about 2 minutes. Remove from heat and stir in lemon juice; let cool 5 minutes.

Add onion mixture and egg to rice and mix well; season generously with salt and pepper. Working with 1 cabbage leaf at a time, cut out the thickest part of the rib. Place 3 heaping Tbsp. filling in the center of the leaf. Fold the leaf around the filling like a burrito.

Arrange cabbage rolls, seam side down, in a single layer in the pot. Add butter and ½ cup water and bring to a simmer over medium-high heat. Reduce heat to low, cover pot, and steam rolls until filling is cooked through and leaves are tender, 18–25 minutes.

Divide cabbage rolls among plates; drizzle with oil and sprinkle with sumac and pepper. Serve with sour cream.